Rep. Tom DeLay (R-Tex.), a primary architect of the Republican majority who became one of the most powerful and feared leaders in Washington, told House allies last night that he will give up his seat rather than face a reelection fight that appears increasingly unwinnable.

The decision came three days after Tony C. Rudy, his former deputy chief of staff, pleaded guilty to conspiracy and corruption charges, telling federal prosecutors of a criminal enterprise being run out of DeLay's leadership offices. Rudy's plea agreement did not implicate DeLay in any illegal activities, but by placing the influence-buying efforts of disgraced Republican lobbyist Jack Abramoff directly in DeLay's operation, the former aide may have made an already difficult reelection bid all but out of reach.

DeLay, who will turn 59 on Saturday, did not say precisely when he would step down, but under Texas law he must either die, be convicted of a felony, or move out of his district to be removed from the November ballot. DeLay told Time magazine that he is likely to change his official residence from Sugar Land, Tex., to Alexandria by the end of May. He said he informed President Bush of his decision yesterday afternoon.

"This had become a referendum on me," he told Time in an article posted on the magazine's Web site last night. "So it's better for me to step aside and let it be a referendum on ideas, Republican values and what's important for this district."

"I'm a realist. I've been around awhile," he added. "I can evaluate political situations." Asked if he had done anything illegal or immoral in public office, DeLay replied: "No."

Former aides and sources close to DeLay said his decision was motivated not by Rudy's guilty plea but by DeLay's concerns that he might lose his suburban Houston seat to his Democratic opponent, former representative Nick Lampson, and his belief that another Republican could win instead.

DeLay's reelection race was expected to be one of the most expensive House campaigns in history and a drain on GOP coffers. The timing of DeLay's resignation could affect how the seat is filled.

Depending on when DeLay steps down, Texas Gov. Rick Perry (R) could call a special election to fill the vacancy. It would be up to local GOP officials to replace DeLay formally on the ballot in November, but party officials say that the winner of a special election -- assuming it is a Republican -- would almost certainly be placed on the fall GOP ballot.

Republicans said that, with DeLay gone, they have a much better chance of holding the seat. Although redistricting took some Republicans out of the district, Bush won 64 percent of the vote there in 2004. According to GOP sources, one almost-certain candidate is Sugar Land Mayor David G. Wallace. Tom Campbell, who was second to DeLay in the primary with 30 percent of the vote, said last night he would run in any special election.

Senior House Republicans have been saying for several weeks that DeLay would make his decision based on what was best for the GOP. Indeed, Democratic House campaign officials have been hoping to face DeLay in November, considering him the weakest Republican candidate they could hope for.

DeLay got a temporary political boost last month when he fended off three challengers to win the Republican primary with 62 percent of the vote. But recent polls showed an uphill climb against Lampson and another former congressman, Steve Stockman, who had cut his ties to the Republican Party to run as an independent. In early January, a Houston Chronicle poll showed DeLay trailing Lampson 30 percent to 22 percent, with Stockman taking 11 percent.

A December poll by CNN, USA Today and Gallup also indicated that a credible Democrat could beat DeLay.